Fourier transform infrared (FTIR) spectrometers are widely used to determine the chemical composition of samples. In an FTIR spectrometer, a beam of infrared radiation is passed through an interferometer that modulates the wavelength content of the infrared beam in a periodic manner. The modulated beam is transmitted to a sample and the infrared light reflected from or transmitted through the sample is directed to a detector. The signal from the detector correlated with the modulation of the beam in the interferometer provides interferogram data that can be analyzed by Fourier transformation to provide the spectrum of absorption or transmission of the infrared light by the sample. These data can then be utilized to determine the chemical composition of the constituents of the sample. FTIR spectrometers can be combined with appropriate infrared microscopes to allow examination of samples on a microscopic level and to determine variations in the composition of the sample over an area of the sample. Examples of prior FTIR microscope systems are shown in U.S. Pat. Nos. 4,877,960, 5,581,085, and 5,864,139, which are incorporated herein by reference.
U.S. Pat. No. 5,864,139 discloses a microspectrometer system having a single confocal aperture used both in transmission and reflection microspectrometry, wherein during a transmission mode of operation as well as the reflection mode the light beam passes through the aperture in both a first direction and a second direction. Thus, a single aperture functions as a confocal aperture for both transmissive and reflective microspectrometry. The microspectrometer system shown in this patent can also be utilized to observe the sample in visible light at the same time that the sample is irradiated with infrared light for spectrometric analysis of the sample.
Typical prior microspectrometer systems have used a single element detector that analyzes all of the infrared light reflected from or transmitted through a selected small area of a sample. The field of view (FOV) of such single element detectors is restricted by the aperture through which the infrared beam passes to ensure that only light from the selected small area of the sample is analyzed. Microspectrometers may also be implemented using array detectors in which the light reflected from or transmitted through a larger area of the sample is imaged on an array formed of multiple individual detector elements so that each detector element views a different portion of the sample. The relatively large field of view required for detector arrays is generally difficult to accommodate in dual aperturing microspectrometer systems. The field of view in dual aperturing systems is limited physically by the position of the spherical mirrors used to focus or collimate the infrared beams. In order to expand the field of view, the incident angle to the spherical mirrors would need to be increased. However, larger incident angles lead to degradation in image quality because of aberrations introduced by off-axis spherical mirror reflections.